Mononuclear aromatic polyisocyanates are well known and are widely used in the preparation of polyurethane and polyurethane/urea elastomers. These mononuclear aromatic diisocyanates generally include compositions such as toluenediisocyanate, phenylenediisocyanate, chlorotoluenediisocyanate and the like. In the preparation of polyurethane and polyurea elastomers the aromatic diisocyanates are reacted with a long chain polyol to produce a prepolymer containing free isocyanate which then may be chain extended with a short chain polyol or aromatic diamine to form a polyurethane or polyurethane/urea elastomer. Long chain polyols, e.g. those having a molecular weight of above 500, are generally utilized to form the prepolymer and the chain extender is a short chain polyol, e.g., C.sub.2 -C.sub.10 glycol, or an aromatic diamine. The long chain polyol provides flexibility and elastomeric properties to the resin, while the short chain polyol or aromatic diamine provides short chain segments to chain extend or crosslink and add toughness and rigidity to the resulting resin.
A major problem with mononuclear aromatic diisocyanates, e.g., toluenediisocyanate (TDI) is that they are toxic and because of their low molecular weight, tend to be quite volatile. Because of their toxicity and volatility, extreme care must be taken in the work place to avoid inhalation and damage to the respiratory tract and contact with the skin. There is some literature on the subject of reacting aromatic diisocyanates, particularly toluenediisocyanate, with short chain polyols to increase their molecular weight and reduce volatility and the utilization of these polyol extended aromatic diisocyanates in the formation of polyurethanes. Representative compositions and processes are noted in the following articles.
U.S. Pat. No. 2,969,386, German Pat. Nos. 756,058 and 870,400 show a method of producing organic polyisocyanates, and particularly urethane polyisocyanates by reacting an aromatic diisocyanate, such as toluene diisocyanate with a short chain diol or triol such as ethylene glycol and trimethylol propane and mixtures with limited side reaction products. The '386 patent noted there were problems associated with the formation of TDI - free (&lt;1%) polyisocyanates and side reactions occurred which lead to the formation of polymeric products. When the isocyanate reactant was increased, the product contained free unreacted toluenediisocyanate. In other prior art processes mixtures of compounds were produced.
The '386 patent attempted to solve the problem associated with the process described in the '058 and '400 patents by producing urethane polyisocyanates substantially devoid of unreacted diisocyanate and having limited side reaction products. Such result was achieved by (a) reacting the organic diisocyanate with polyol in an amount in excess over that stoichiometrically required, e.g., 2:1 molar ratio at a temperature above the melting point of the mixture but low enough, i.e., below 100.degree. C., to avoid undesirable polymerization reactions; (b) extracting the resulting resin from the reaction mixture by contacting it with an aliphatic or acyclic hydrocarbon; and then, (c) separating the solvent layer which contains unreacted diisocyanate and polyol. By this technique one was able to obtain higher molecular weight urethane linked aromatic diisocyanates free of unreacted diisocyanate which then could be used for making polyurethanes.
Examples in the '386 patent show reacting a 65:35 mixture of 2,4- and 2,6-toluenediisocyanate with a polyol mixture of 1,3-butylene glycol, trimethylolpropane, and a polyester polyol containing 4.98% OH in a mole ratio of 3.70:0.18:0.28:0.0015, respectively, or 200% excess toluenediisocyanate in the presence of solvent at a temperature of 80.degree. C. for a period of about 2 hours. After removal of unreacted toluenediisocyanate, a light amber solid soluble in ethyl acetate was prepared.
U.S. Pat. No. 3,285,951 discloses the preparation of adducts of 2,4-toluenediisocyanate and 2,3-butanediol containing 85% mesoisomer, with the mole ratio of toluenediisocyanate to butanediol being at least 2:1 preferably 2-4:1, for use in preparing polyurethanes. In one example. a product was prepared by dissolving toluenediisocyanate in a suitable solvent, such as hexane, stirring at high speed and adding butanediol to the solution. As the reaction was carried out, a solid white powder precipitated. The precipitate then was reacted with a millable gum of polypropylene glycol, butanediol and toluenediisocyanate.
U.S. Pat. No. 3,218,348, discloses the preparation of stable urethane polyisocyanate solutions by the sequential reaction of a polyisocyanate e.g., toluenediisocyanate with polyol mixture. To avoid crystallization on standing the polyisocyanate was reacted with a triol and then the resulting intermediate product was reacted with a diol. The patentees noted that when the procedure set forth in the '396 patent described above, was carried out adding the diol first, then the triol or simultaneously adding the diol and triol to the isocyanate in the presence of organic solvent, the reaction product was unstable in the solution and would crystallize within a matter of minutes or days.
U.S. Pat. No. 3,076,770 discloses a process for producing low density cellular polyurethane foams by the reaction of an organic polyisocyanate with short chain polyols and utilizing the sequential addition of a triol and diol as in U.S. Pat. No. 3,218,348. Typically, high molecular weight polyols along with trifunctional polyols had been used for foams. The cellular polyurethanes were prepared by first reacting an isocyanate with a diol/triol mixture and removing free isocyanate from the reaction product. The reaction product was dissolved in a solvent and then reacted with a polyester polyol to form polyurethane systems.
U.S. Pat. No. 3,020,249 discloses a process for preparing rigid and semi-rigid polyurethane foams from an alkyd polyester resin and a diisocyanate containing reaction product derived from the reaction of toluenediisocyanate and 1,2,6-hexanetriol. The hexanetriol containing excess diisocyanate adducts were formed by reacting toluenediisocyanate with hexanetriol containing excess diisocyanate at temperatures of 100.degree.-120.degree. C.; the toluenediisocyanate was included in substantial excess.
U.S. Pat. No. 3,023,228 discloses a process for producing solid low molecular weight urethane polyisocyanate/urea systems by reacting a diisocyanate e.g., toluenediisocyanate, with a mixture of a short chain diol, e.g. butanediol, and water in the presence of a solvent, e.g., acetonitrile. Temperatures from about 10.degree.-35.degree. C. are suggested as being suited for forming the reaction product. An example shows reacting toluenediisocyanate with ethylene glycol in the presence of water and acetone (solvent) for about one to two hours at which time the reaction mixture solidifies and a crystal magma forms. Another example described the reaction of toluenediisocyanate with diethylene glycol and water to produce a product having a softening point of 155.degree. C. and an isocyanate content of 23.5%. The resulting low molecular weight products having an isocyanate content typically from 18-25%, are valuable as reactants in the production of polyurethane plastics.